User: Data conversion

[iLLiCiTiT]

Changelog Description

Convert parse json string values and convert ownAttrib on user to same structure as on other entities.

Testing notes:

1. Function get_users does not require accessGroups in fields if are customized.
2. Values of accessGroups and allAttrib are converted from json string to python object when received using get_users.
3. Function get_user does fill ownAttrib the same way as other entities (making it a dictionary with None values for those which don't have own values on user).